Studies On Environmental Photochemical Transformation Of Triazole Chiral Fungicides And Their Photo-induced Toxicity

Pesticides are widely used in agricultural production and play a key role in controlling pests and increasing crop yields,which also causes serious environmental pollution at the same time.Pesticides are frequently detected in various environmental media and have been concerned widely due to environmental persistence,adverse effects on organisms and the environment.Photochemical transformation is one of the important abiotic transformation pathways in the environment,which significantly affects their environmental fate and persistence.About 30%of commercial pesticides are chiral compounds,and 93%of them are racemates for sale and use.At present,most of the photochemical behavior studies of chiral pesticides focus on the racemic level.Whether the photochemical transformation of chiral enantiomers in different environmental media is stereoselective needs to be further explored.Photolysis behavior and the photo-induced toxicity of triazole chiral fungicides in different media of aquatic organisms have rarely been reported at the enantiomeric level.In this thesis,the photolysis kinetics,mechanism,pathway and photo-induced toxicity of four triazole fungicides and their enantiomers in water,liquid-solid or air-solid heterogeneous systems were investigated.And the effects of dissolved organic matter（DOM）and anions/cations in natural water on the photolysis of pesticides were unveiled.The main findings are as follows:（1）The photochemical behavior and photo-induced toxicity of triadimefon（TF）and triadimenol（TN）in water were studied.TF has two isomers（R-TF and S-TF）with one chiral center,while TN has two chiral centers with four isomers（RS-TN and SR-TN,RR-TN,and SS-TN）.Experiments showed that the photolysis rate(k_obs)of TF had no enantioselectivity,but the kobs of TN exhibits enantioselectivity:kobs（RS-TN）=kobs（SR-TN）>kobs（RR-TN）=kobs（SS-TN）.The two pesticides have different photolysis mechanisms.The photolysis of TF occurs mainly through its excited singlet state,while the photolysis of TN occurs mainly through its excited triplet state.Their photodegradation pathways mainly include dichlorination,elimination of triazole and cleavage of ether bond.Due to their similar structure,some products have been observed during the degradation of both TF and TN,such as p-chlorophenol,hydroquinone and triazole.Toxicity experiments showed that some intermediate products with higher toxicity than the parent compound were produced.Both TF and TN exhibited photo-induced toxicity to Vibrio fischeri.In addition,TN exhibits enantioselective photoinduced toxicity after 240 min irradiation,which can be attributed to the formation of chiral products.Although Mg²⁺and NO^3-can promote the photodegradation of TF and TN,their promotion effect is negligible due to their low concentration in natural water.Due to the shading-effect of dissolved organic matter（DOM）and suspended particulate matter in natural water,their photodegradation rate in natural water is slower than that in pure water.（2）The photochemical behavior and photo-induced toxicity of prothioconazole（PTCZ）was investigated in water and solid particles:The photolysis rate of PTCZ in pure water is much lower than that of TN and TF,which means that it has higher persistence in the environment.The photolysis of PTCZ in pure water is mainly dominated by direct photolysis triggered by excitation of triplet state,and·OH has a certain contribution to its photolysis.As a photosensitizer by producing ¹O₂ and·OH,DOM can significantly promote the degradation of PTCZ at a rate of about 3 times that in pure water,initiating indirect photolysis of PTCZ.In addition,the degradation rates in different systems are:k _DOM>k _SG>k_water>k _CLL>k _CD.Air humidity and loading ratio on solid particles can affect the photolysis rate of PTCZ.There is no enantiomeric selectivity on the studed solid-air systems.The main degradation pathways of PTCZ include desulfurization,dechlorination and hydroxylation,resulting in low molecular weight dehalogenation products and high molecular weight hydroxylation products.The toxicity experimental results showed that PTCZ produced some higher toxic intermediates during the degradation process,resulting in photo-induced toxicity to Vibrio fischeri.（3）The photochemical behavior and photo-induced toxicity of mefentrifluconazole（MFFZ）were explored in water,solid-air and solid-water systems.The photodegradation rate of MFFZ is faster than that of the above triazole pesticides.The direct photolysis in pure water of MFFZ mainly occurs by excitation of the triplet state.The excited state MFFZ can also sensitize dissolved oxygen to produce ¹O₂ and·OH to participate in photolysis.Although NO^3-can accelerate the photolysis of MFFZ,its promotion effect is negligible due to low concentration in natural water.k_obs of MFFZ in natural water slower than that in pure water can be can be attributed to the shading-effect of the DOM.The main photolysis pathways of MFFZ include defluorination,dechlorination and cleavage of C-N bonds,thereby generating dechlorination products,hydroxylation products,triazoles and other products.In addition,the degradation rates of MFFZ in different systems is:the fastest pure water system,followed by solid-water system,and finally solid-air system,indicating that MFFZ has relatively high durability in plants or soil.It is more conducive to its degradation after migrating to the water environment.Due to the enantioselective adsorption of MFFZ enantiomers on the CLL surface,which in turn affects their distribution in solids and water,the degradation rates of R-MFFZ and S-MFFZ are different.MFFZ has no obvious photo-induced toxicity in pure water due to the low limitation of solubility.However,when it is loaded on the surface of CLL,toxic products gradually accumulate with the increase of light time in solid-water system,resulting in photo-induced toxicity to Vibrio fischeri.（4）Based on photolysis kinetics and mechanisms of 11 pesticides,a classified QSAR model was established.The photolysis rate constants of the selected 11 pesticides varied widely.The photolysis rates of nitro-substituted neonicotinoids and butenolide insecticides are much faster than that of cyanoimino-substituted neonicotinoids and sulfoximine insecticides.The photolysis in pure water of 7 pesticides（nitenpyram,imidacloprid,clothianidin,imidaclothiz,thiamethoxam,dinotefuran and sulfoxaflor）was dominated by direct photolysis,and a single descriptor,the highest molecular orbital energy(E_HOMO)or chemical potential（μ）has a good correlation with log k.The photolysis of the other four pesticides is relatively complex,including self-sensitization photolysis with the participation of ¹O₂ and·OH,so the QSAR model of the 11 pesticides contains two descriptors of E_gap(E_LUMO-E_HOMO)and dipole moment（δ）.Classified QSAR models show that the photolysis rate of pesticides was correlated with E_LUMO,E_HOMO,μandδ,which well explained the photolysis mechanism of 11 pesticides.In summary,the photochemical conversion behavior of TF,TN,PTCZ and MFFZ in water,liquid-solid and air-solid systems was studied.The four triazole pesticides studied could all undergo photodegradation,and their k_obs varied greatly in different systems.Except for TF,the photolysis of other triazole pesticides in water is dominated by direct photolysis triggered by excitation of their triplet states,and the photodegradation is mainly through dehalogenation and hydroxylation.Silica and cellulose can induce the production of ¹O₂ and·OH with light,which in turn oxidize organic pollutants.The research results can deepen the understanding and cognition of the environmental risks of these compounds,and provide a more convincing scientific basis and support for the scientific and rational use of pesticides and the formulation and implementation of policies.